Metal alloy catalysts for fuel cell cathodes

ABSTRACT

A metal alloy catalyst for the oxygen reduction reaction in fuel cells is disclosed. The catalyst contains the metals Pd, M1 and M2. M1 and M2 are different metals selected from Co, Fe, Au, Cr and W, excluding the combination PdCoAu.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/088,761, filed on Jun. 9, 2008, which is a national phase ofInternational Application No. PCT/GB2006/050319, filed on Oct. 6, 2006,which claims the benefit of priority to GB Application No. 0520473.0,filed on Oct. 7, 2005. The content of the prior applications areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention is concerned with metal alloy catalysts for the oxygenreduction reaction (ORR) that takes place at the cathodes of protonexchange membrane (PEM) fuel cells.

BACKGROUND OF THE INVENTION

A fuel cell comprises a fuel electrode (anode), an oxidizer electrode(cathode), an electrolyte interposed between the electrodes and meansfor separately supplying a stream of fuel and a stream of oxidizer tothe anode and the cathode, respectively. In operation, fuel supplied tothe anode is oxidized releasing electrons which are conducted via anexternal circuit to the cathode. At the cathode the supplied electronsare consumed when the oxidizer is reduced. Proton exchange membrane fuelcells use a solid proton-conducting polymer membrane as the electrolyte.

Conventional fuel cells use hydrogen gas as the fuel. Pure hydrogen gas,however, is difficult and costly to supply. Thus, hydrogen gas istypically supplied to a fuel cell using a reformer, which steam-reformsmethanol and water to a hydrogen-rich fuel gas containing carbondioxide. Theoretically, this “reformate” gas consists of 75 vol. %hydrogen and 25 vol. % carbon dioxide. In practice, however, this gasalso contains nitrogen, oxygen and, depending on the degree of purity,varying amounts of carbon monoxide. This process is complex, and theconversion of a liquid fuel directly into electricity would bedesirable, as then a high storage density, system simplicity andretention of existing fueling infrastructure could be combined. Methanolis an especially desirable fuel because it has a high energy density,low cost and is produced from renewable resources. Thus, there is now astrong interest in the direct methanol fuel cell, in which the overallprocess that occurs is methanol and oxygen react to form water andcarbon dioxide and electricity.

Conventionally platinum has been used as the cathode catalyst for ORR inPEM fuel cells. Because of the high cost of platinum, there has beeninterest to find non-platinum catalysts which will have comparable orincreased activity relative to platinum. Palladium has been proposed asan alternative to platinum, because palladium is available at lowercost. Palladium-cobalt binary alloys have shown useful activity for ORR.However the present inventors have found that the stability of suchalloys is less than is desired in practical applications for PEM fuelcells.

It is an object of this invention to provide palladium-based ternaryalloys which are effective catalysts for ORR and which have improvedstability relative to palladium-based binary alloys.

SUMMARY OF THE INVENTION

According to the present invention there is provided a metal alloycatalyst for the oxygen reduction reaction in proton exchange membranefuel cells, the alloy containing the metals Pd, M1 and M2 where M1 andM2 are different metals selected from Co, Fe, Au, Cr and W; butexcluding the combination PdCoAu.

In the alloys of this invention, in general one of M1 and M2 is anactivating metal which increases the activity relative to Pd atone, andthe other of M1 and M2 is a stabilising metal, or forms a stabilisingmetal combination M1M2, which improves the stability of the alloy forfuel cell use.

Preferably the catalyst is an alloy consisting essentially of the metalsPd, M1 and M2 where M1 and M2 are different metals selected from Co, Fe,Au, Cr and W (but excluding the combination PdCoAu).

In other words, the catalyst is preferably a ternary alloy consisting ofthe metals Pd, M1 and M2 where M1 and M2 are different metals selectedfrom Co, Fe, Au, Cr and W and incidental inevitable impurities (butexcluding the combination PdCoAu).

In another aspect the present invention provides a cathode for a PEMfuel cell comprising a cathode support and a catalyst of composition asdefined above.

In a further aspect, the present invention provides a membrane-electrodeassembly for a fuel cell comprising a proton exchange membrane, an anodeand a cathode, in which the cathode includes an alloy catalyst ofcomposition as defined above.

The invention also provides a fuel cell comprising at least onemembrane-electrode assembly of a proton exchange membrane, an anode anda cathode which includes an alloy catalyst of composition as definedabove.

DESCRIPTION OF THE FIGURES

FIG. 1 shows the entire set of PdCo and PdCoAu compositions depositedand screened for ORR in BINARY EXAMPLEs 1-3 and TERNARY EXAMPLEs 1-5;

FIG. 2 shows a plot of the steady-state currents for the reduction ofoxygen for the PdCo binary system in 0.50 M HClO₄ (aq) at (a) 0.70 V and(b) 0.80 V vs. RHE, based on the data of BINARY EXAMPLE 1;

FIG. 3 plots (a) the data of surface area for the PdCo binary systemestimated from the CO(ads) oxidation charge and (b) data of the specificcurrent densities for the reduction of oxygen at 0.70 V vs. RHE in 0.50M HClO₄ (aq) based on the data of BINARY EXAMPLE 2;

FIG. 4 shows the decrease in the surface oxide reduction peak currentfor the PdCo binary system as measured from cyclic voltammogramsrecorded before and after the steady state oxygen reduction screeningexperiments in 0.50 M HClO₄ (aq), based on the data of Binary Example 3;

FIG. 5 plots the steady-state oxygen reduction currents for a PdCoAuternary array at (a) 0.70 V, (b) 0.75 V, and (c) 0.80 V vs. RHE in 0.5 MHClO₄ (aq), as discussed in Ternary Example 1. The z-axes in each caseare in μA;

FIG. 6 plots the specific current densities for a PdCoAu ternary arrayfor the steady-state oxygen reduction (5 a) at 0.70 V, (5 b) at 0.75 V,and (5 c) at 0.80 V vs. RHE in 0.5 M HClO₄ (aq), as discussed in TERNARYEXAMPLE 2. The z-axes in each case are in mA cm⁻²;

FIG. 7 plots the steady-state oxygen reduction currents at (a) 0.70 V,(b) 0.75 V, and (c) 0.80 V vs. RHE in 0.5 M HClO₄ (aq) for the entirePdCoAu binary and ternary phase space investigated, as discussed inTERNARY EXAMPLE 3. The z-axes in each case are in μA;

FIG. 8 plots the specific current densities for the steady-state oxygenreduction at (a) 0.70 V, (b) 0.75 V, and (c) 0.80 V vs. RHE in 0.5 MHClO₄ (aq) for the entire PdCoAu binary and ternary phase spaceinvestigated, as discussed in TERNARY EXAMPLE 4. The z-axes in each caseare in μA cm⁻²;

FIG. 9 plots (a) the change in the surface oxide reduction peak currenttaken from cyclic voltammograms recorded before and after oxygenreduction screening in 0.5 M HClO₄ (aq), (b) the same data as (a) butwith the binary data removed to allow rescale for smaller currentchanges as discussed in TERNARY EXAMPLE 5. The in z-axes in each caseare in μA;

FIG. 10 plots steady-state currents for the reduction of oxygen for thePdFe binary system in 0.50 M HClO₄ (aq) at 0.70 V vs. RHE, as discussedin BINARY EXAMPLE 4;

FIG. 11 shows composition of all PdCr and PdCoCr thin films prepared bydeposition and electrochemically screened in BINARY EXAMPLEs 5 and 6 andTERNARY EXAMPLEs 7-9;

FIG. 12 plots current vs. composition for the reduction of oxygen at0.70 V vs. RHE in 0.5 M HClO₄ (aq) for the PdCr binary system, asdiscussed in BINARY EXAMPLE 5;

FIG. 13 plots current vs. composition for ORR at 0.80 V vs. RHE in 0.5 MHClO₄ (aq) for the PdCr binary system, as discussed in BINARY EXAMPLE 5;

FIG. 14 plots current vs. composition for ORR at 0.70 V vs. RHE in 0.5 MHClO₄ (aq)+0.5 M methanol for the PdCr binary system, as discussed inBINARY EXAMPLE 6;

FIG. 15 plots current vs. composition for ORR at 0.80 V vs. RHE in 0.5 MHClO₄ (aq)+0.5 M methanol for the PdCr binary system, as discussed inBINARY EXAMPLE 6;

FIG. 16 plots the steady state oxygen reduction currents for the PdCoCrfilms and associated binary compositions at 0.70 V vs. RHE in 0.5 MHClO₄ (aq), as discussed in Ternary Example 7;

FIG. 17 plots the steady state oxygen reduction currents for the PdCoCrfilms and associated binary compositions at 0.75 V vs. RHE in 0.5 MHClO₄ (aq), as discussed in Ternary Example 7;

FIG. 18 plots the steady state oxygen reduction currents for the PdCoCrfilms and associated binary compositions at 0.80 V vs. RHE in 0.5 MHClO₄ (aq), as discussed in Ternary Example 7;

FIG. 19 shows the electrochemical surface area of PdCoCr system, asdiscussed in TERNARY EXAMPLE 8. The areas were estimated from the COoxidation charge as explained in BINARY EXAMPLE 2;

FIG. 20 plots the steady-state oxygen reduction specific currentdensities for the entire binary and ternary phase space of PdCoCrinvestigated at 0.70 V vs. RHE in 0.5 M HClO4(aq), as discussed inTERNARY EXAMPLE 9;

FIG. 21 plots the steady-state oxygen reduction specific currentdensities for the PdCoCr and associated binary compositions at 0.80 Vvs. RHE in 0.5 M HClO₄ (aq), as discussed in TERNARY EXAMPLE 9;

FIG. 22 indicates (the hatched area) regions of special interest in thePdCoCr system, based on the data of TERNARY EXAMPLEs 7 to 9;

FIG. 23 shows the composition in At. % of PdCo, PdW and PdCoW thin filmsdeposited and electrochemically screened in TERNARY EXAMPLEs 10-13;

FIG. 24 is a plot of the steady state oxygen reduction currents for thePdCo, PdW and PdCoW films at 0.70 V vs. RHE in 0.5 M HClO4(aq), asdiscussed in TERNARY EXAMPLE 10;

FIG. 25 is a plot of the steady state oxygen reduction currents for theentire PdCoW binary and ternary phase space investigated at 0.80 V vs.RHE in 0.5 M HClO₄ (aq), as discussed in TERNARY EXAMPLE 10;

FIG. 26 shows the electrochemical surface area of the PdCoW system, asdiscussed in TERNARY EXAMPLE 11. The areas were estimated from the COoxidation charge as explained in BINARY EXAMPLE 2;

FIG. 27 is a plot of the steady-state oxygen reduction current densitiesfor the PdCo, PdW and PdCoW films at 0.70 V vs. RHE in 0.5 M HClO₄ (aq),as discussed in TERNARY EXAMPLE 12;

FIG. 28 is a plot of the steady-state oxygen reduction current densitiesfor the PdCo, PdW and PdCoW films at 0.80 V vs. RHE in 0.5 M HClO₄ (aq),as discussed in TERNARY EXAMPLE 12;

FIG. 29 shows the composition change of a PdCoW ternary array after anelectrochemical screening as measured by EDS before and after screening,as discussed in TERNARY EXAMPLE 13;

FIG. 30 shows (hatched area) regions of special interest for the PdCoWsystem;

FIG. 31 shows the composition of PdFe, PdCr and PdFeCr thin filmsdeposited and electrochemically screened in TERNARY EXAMPLEs 14-16;

FIG. 32 is a plot of the steady state oxygen reduction currents for thePdFe, PdCr and PdFeCr films at 0.70 V vs. RHE in 0.5 M HClO₄ (aq), asdiscussed in TERNARY EXAMPLE 14;

FIG. 33 is a plot of the steady state oxygen reduction currents for theentire PdFeCr binary and ternary phase space investigated at 0.80 V vs.RHE in 0.5 M HClO₄ (aq), as discussed in TERNARY EXAMPLE 14;

FIG. 34 shows the Electrochemical surface area of PdFeCr system, asdiscussed in TERNARY EXAMPLE 15;

FIG. 35 is a plot of the steady-state oxygen reduction specific currentdensities for the PdFe, PdCr and PdFeCr films at 0.70 V vs. RHE in 0.5 MHClO₄ (aq), as discussed in Ternary Example 16;

FIG. 36 is a plot of the steady-state oxygen reduction specific currentdensities for the PdFe, PdCr and PdFeCr films at 0.80 V vs. RHE in 0.5 MHClO₄ (aq), as discussed in Ternary Example 16;

FIG. 37 shows (hatched area) regions of special interest for the PdFeCrsystem;

FIG. 38 shows the specific current densities (corrected for masstransport corrected and real surface area of Pd) for the oxygenreduction reaction at 0.85 V and 0.90 V vs. RHE in 0.5 M HClO₄ (aq) at25.degree. C. for the catalysts deposited on rotating disc electrodes.Data obtained with thin films of pure Pt and Pd are shown forcomparison; as discussed in Reference Example 1;

FIG. 39 shows the specific activity for ORR per market price in $ at0.85 V vs. RHE of all the studied catalysts. Values for pure platinumand palladium are included for comparison.

DESCRIPTION OF THE INVENTION

In its broadest aspect, the present invention is a metal alloy catalystfor the oxygen reduction reaction in fuel cells, the alloy comprisingthe metals Pd, M1 and M2, where M1 and M2 are different metals selectedfrom Co, Fe, Au, Cr or W; but excluding the combination PdCoAu.

The present invention has been developed by depositing and screeningthin films of binary and ternary alloys containing palladium, M1 and/orM2, using the “Physical Vapour Deposition Method for the High-ThroughputSynthesis of Solid-State Material Libraries” disclosed by Guerin et alin J. Comb. Chem. 2006, 8, 66 and using the screening method disclosedby Guerin et al in “Combinatorial Electrochemical Screening of Fuel CellElectrocatalysts” in J. Comb. Chem. 2004, 6, 149 and “High-ThroughputSynthesis and Screening of Ternary Metal Alloys for Electrocatalysis” inJ. Phys. Chem. B, 2006, 110, 14355. As indicated in the Examples below,by use of this screening technique binary alloys of composition Pd andM1 or M2 can be optimised for the ORR; also it can be determined whereinclusion of M2 or M1 into the optimum binary alloy compositionsprovides an alloy containing PdM1M2 with effective ORR activity,preferably superior to Pd alone, and with improved stability. As aresult, the metal alloys suitable for use as catalysts for the cathodesof PEM fuel cells are identified.

In particular, the present inventors have established that binary alloysof composition PdM1 where M1 is Co or Fe can be optimised for the ORR,and that inclusion of M2 where M2 is Au, Cr, W or Fe into the optimumbinary alloy compositions provides an alloy containing PdM1M2 with ORRactivity superior to Pd alone and with improved stability.

In one preferred group of catalysts M1 is Fe and M2 is Au, Cr, Co or W.

For the catalyst composition PdFeAu, a region of special interest foreffective ORR has from 50-70 At. % Pd, 50-30 At. % Fe and up to 20 At. %Au.

For the catalyst composition PdFeCr, a region of special interest foreffective ORR has from 30-80 At. % Pd, 20-70 At. % Fe and up to 40 At. %Cr. Compositions of special interest for the system PdFeCr are alsofound in the hatched area of the ternary diagram in FIG. 37.

The binary catalyst PdFe, and use thereof as a catalyst in fuel cells,forms a further aspect of this invention.

In another preferred group of catalysts M1 is Co and M2 is Cr or W.

For the catalyst composition PdCoCr, a region of interest for highactivity has a composition range of 30 to 80 At. % Pd; the high activityis maintained at up to 30 At. % Cr. Another region of interest is at30-60 At. % Pd and 30-70 At. % Co and 0-20 At. % Cr. However a morestable region appears with the amount of Co decreased to 10-30 At. % andthe amount of Cr increased to 20-40 At. %. Compositions of specialinterest for the system PdCoCr are also found in the hatched area of theternary diagram shown in FIG. 22.

For the catalyst PdCoW, a region of interest for high activity has acomposition range of 30 to 80 At. % Pd. The high activity is maintainedat up to 20 At. % W. However a more stable region appears with theamount of Pd increased to 60-80 At. % and the amount of Co less than 40At. %. The most active alloys contain 20-60 At. % Pd, 30-70 At. % Co and0-30 At. % W, while more stable alloys are composed of more than 60 At.% Pd, less than 40 At. % Co and less than 20 At. % W. Compositions ofspecial interest for the system PdCoW are also found in the hatched areaof the ternary diagram shown in FIG. 30.

The binary alloy catalysts PdCr and PdW, and use thereof as catalysts infuel cells, form another aspect of this invention.

The alloys of the present invention, may be used in proton exchangemembrane fuel cells in which oxygen is electrochemically reduced at thecathode. Typical cells include hydrogen/oxygen fuel cells, hydrogen/airfuel cells, and direct liquid fuel cells including DMFC (Direct MethanolFuel Cells) with protonic electrolytes.

Ternary alloys suitable for use as catalysts for fuel cell electrodes inaccordance with the present invention may be selected from the groupconsisting of PdCoCr, PdCoW, PdFeCr, PdFeW, PdCrW, PdWAu, PdCrAu, PdCoFeand PdFeAu.

In general, the ternary cathode catalysts and cathode electrodes of thepresent invention may be used in fuel cells wherein the anode reactioninvolves catalytic oxidation of any fuel containing hydrogen e.g.,hydrogen and reformated-hydrogen fuels and hydrocarbon-based fuels.Applicable hydrocarbon-based fuels include saturated hydrocarbons suchas methane (natural gas), ethane, propane and butane; waste-tip off-gas;oxygenated hydrocarbons such as methanol and ethanol; and fossil fuelssuch as gasoline and kerosene; and mixtures thereof. The preferred fuel,in view of the fact that the ORR activities of the Pd-based catalysts ofthis invention are not affected by methanol crossover, is methanol.

In the case of DMFC, methanol crossover degrades the catalytic activityof ORR when Pt is used as the cathode catalyst. On the other hand, theORR activity of Pd and Pd alloy catalyst are not affected by methanolcrossover. Therefore, when used in DMFC, the catalysts of the presentinvention are superior to Pt catalyst from the viewpoint of not only thecost but also the cell performance.

Accordingly an advantageous aspect of the invention is a DMFC in whichternary alloys of the invention are used as catalysts for ORR.

In the screening methods used in the present invention to assess theproperties of the ternary alloys, the alloys are prepared by blendingthe component metals using the techniques disclosed in WO 2005/035820,the entire disclosure of which is incorporated herein by reference. Forpreparation of alloys on a larger scale for use in cathodes,conventional alloy preparation techniques which will be familiar tothose skilled in this technology may be used, such as sputtering,reduction of metal oxide mixtures, reduction of mixed salts depositedfrom solutions and other known techniques may be used. The resultantalloys may contain incidental or inevitable impurities arising from theproduction process, so far as these do not affect the desired activity.

Some typical preparation methods that may be used are described bySinfelt in Ann. Rev. Mat. Sci., 1972, 2, 641 “Highly Dispersed CatalyticMaterials” and more recently by Chan et al. in J. Mater. Chem., 2004,14, 505 “Supported Mixed Metal Nanoparticles as Electrocatalysts in LowTemperature Fuel Cells”. There are of course many alternate methods thatmay be used or envisaged by persons skilled in the art of dispersedcatalyst synthesis.

The alloy catalysts may be deposited directly on the surface of a protonexchange membrane for contact with a current collector. Alternativelythe alloy catalysts may be deposited on the surface of a cathodesupport, or within the pores of a porous cathode support, such as acarbon structure that is placed in contact with the membrane.

Carbon supported catalysts are normally used for fuel cells becausesurface area of catalysts is greatly increased by using carbon supportand carbon has sufficient electronic conductivity and chemical stabilityunder fuel cell operating conditions. The preparation of dispersed alloyon carbon supports may be achieved in a number of ways. For instance,the alloy catalysts may be formed by reduction of a mixture of compoundsof component metals, or heat treatment of carbon supported Pd whereinthe other metal salts are precipitated or adsorbed onto the surface.Alternatively, the alloy particles may be formed on the carbon supportsby physical deposition, such as sputtering, physical evaporation andchemical vapour deposition.

The proton exchange membrane (PEM) is typically a polymeric ion exchangemembrane, especially a perfluorinated ionomer membrane such asperfluorosulfonated membranes, for example the commercially availableNafion™ membrane and its derivatives produced by du Pont. Nafion™ isbased on a copolymer made from tetrafluoroethylene andperfluorovinylether, and is provided with sulfonic groups working asion-exchanging groups. Other suitable proton exchange membranes areproduced with perfluorinated monomers such as octafluorocyclobutane andperfluorobenzene.

The membrane electrode assemblies (MEA), which also include an anode,having a hydrogen oxidation or liquid fuel oxidation catalyst structure,on the opposite surface of the membrane, may be assembled in series asMEA stacks to form fuel cells.

The fuel cells include means for supplying oxygen or anoxygen-containing gas such as air to the cathode catalyst for ORR andmeans for supplying a hydrogen-containing gas or liquid such as methanolto the anode catalyst for generation of protons. Typically air issupplied to the cathode and pure hydrogen to the anode. Gas supplychannels may be formed within porous cathode and anode supportstructures, or gas flow field plates may be placed in contact with thecathode(s) and anode(s).

The detailed construction of PEM fuel cells is well known to thosefamiliar with such technology and does not form part of the inventivesubject matter of this invention as such. Typically the fuel cellcomprises an anode, a cathode, a proton exchange membrane between theanode and the cathode, and catalysts for the catalytic oxidation of ahydrogen-containing fuel and for the reduction of oxygen.

A typical direct methanol fuel cell (DMFC) has a methanol electrode(fuel electrode or anode) and an air electrode (oxidizer electrode orcathode). In between the electrodes, a proton exchange membrane servesas an electrolyte. The proton exchange membrane, the anode and thecathode are generally integrated into one body, so there is no contactresistance between the electrodes and the proton exchange membrane.Electricity is generated by methanol oxidation by introducing methanolinto a methanol fuel chamber open to the anode, while oxygen, preferablyas air, is introduced into an air chamber open to the cathode. Themethanol is oxidised at the anode to produce carbon dioxide gas,hydrogen ions and electrons. An electric current is withdrawn from thefuel cell into an outer circuit by current collectors in contact withthe electrodes. Hydrogen ions migrate through the acidic proton exchangemembrane and react with oxygen and electrons from the outer circuit atthe cathode to form water. The methanol may be introduced as a dilutesolution, which may be acidic, to enhance the chemical reaction andincrease power output.

The invention and its efficacy are further illustrated in the followingExamples. The Examples detail experiments where the samples of thin filmalloys were deposited using the techniques of WO 2005/035820 andassessed by the HT-PVD technique disclosed by Guerin et al in J. Comb.Chem. 2006, 8, 66, “Physical Vapour Deposition Method for theHigh-Throughput Synthesis of Solid-State Material Libraries”. The entiredisclosure of both documents is incorporated herein by reference. Thesubstrates for electrochemical screening were electrochemical arraysconsisting of a 10 by 10 arrangement of gold electrodes on a siliconnitride wafer substrate.

High-throughput electrochemical screening (HT-ES) consists of recordingthe current at all 100 electrodes on an array simultaneously (pseudoparallel data acquisition). This is achieved by means of a 100 channelcurrent follower, a common potential control for all electrodes and aspecifically designed data acquisition software, as disclosed by Guerinet al. in J. Comb. Chem. 2004, 6, 149, “Combinatorial ElectrochemicalScreening of Fuel Cell Electrocatalysts” and in J. Phys. Chem. B, 2006,110, 14355, “High-Throughput Synthesis and Screening of Ternary MetalAlloys for Electrocatalysis”.

The ORR testing described in the examples provides an effective modelfor the suitability of alloy samples as the catalyst at the cathode of aPEM fuel cell. During the steady state ORR experiments the potential wasapplied in steps from 0.7 to 0.9, then back to 0.7 V vs. a reversiblehydrogen electrode (RHE) using 50 mV steps at intervals of 90 s perstep. Oxygen gas was bubbled through the electrolyte throughout theexperiment. All experiments were carried out in 0.5 M HClO₄ (aq)electrolyte and at room temperature (20° C.).

During cyclic voltammetry experiments the potential was cycled between0.4 and 1.2 V vs. RHE at 50 mV s.sup.−1. All experiments were inoxygen-free 0.5 M HClO₄ (aq) electrolyte and at room temperature (20°C.).

PREPARATION EXAMPLE 1

For assessment of the PdCoAu system 3 ternary PdCoAu samples wereprepared using the techniques of WO 2005/035820. The compositions of thesamples were measured using energy dispersive spectroscopy (EDS) and arequoted as atomic percents (At. %). The range of compositions preparedwere: (1) At. % Pd (7.6 to 73.1), At. % Co (5.1 to 65.0), and At. % Au(16.5 to 47.2), (2) At. % Pd (12.9 to 89.9), At. % Co (0.2 to 70.4), andAt. % Au (6.7 to 50.9), and (3) At. % Pd (2.5 to 98.1), At. % Co (0 to93.5), and At. % Au (0 to 63.0). The range of compositions prepared forthe binary PdAu and PdCo samples were (4) At. % Pd (9.4 to 95.5) and At.% Au (4.5 to 90.6) and (5) At. % Pd (31.8 to 99.7) and At. % Co (0.3 to68.2). For the screened samples in the PdCoAu ternary and binary phasespaces, all the compositions tested are plotted on FIG. 1 in atomicpercentages. The activity of the AuCo binary space was not screened forORR activity, as it is not likely that this area of the ternarycomposition space will show any activity at reasonable operatingpotentials. In addition the activity of the pure Pd component wasdetermined using an array composed of 100 identical Pd electrodes. Intotal 600 thin film samples were prepared and screened for oxygenreduction activity in order to populate the composition activity spacefor the PdCoAu alloy system.

PREPARATION EXAMPLE 2

For the PdCoCr alloy system, in total 1100 thin film samples weresimilarly prepared and screened for oxygen reduction activity in orderto populate the composition activity space. The range of compositionsprepared and tested is shown in FIG. 11

PREPARATION EXAMPLE 3

For the PdCoW system, in total 700 thin film samples of PdCo, PdW andPdCoW alloys were prepared and screened for oxygen reduction activity.The compositions of the samples were measured using energy dispersivespectroscopy (EDS) and are quoted as Atomic %. FIG. 23 shows the entireset of binary and ternary compositions deposited and screened for oxygenreduction reaction.

PREPARATION EXAMPLE 4

For the PdFeCr system in total 700 thin film samples of PdFe, PdCr andPdFeCr alloys were prepared and screened for oxygen reduction activity.The compositions of the samples were measured using energy dispersivespectroscopy (EDS) and are quoted as Atomic %. FIG. 31 shows the entireset of binary and ternary compositions deposited and screened for oxygenreduction reaction.

BINARY EXAMPLE 1

The steady state current for oxygen reduction for the PdCo binary systemat 0.70 V and 0.80 V vs. RHE was measured. FIG. 2 shows the steady statecurrents for oxygen reduction at 0.70 V (FIG. 2 a) and 0.80 V (FIG. 2 b)vs. RHE. The data shows there was a clear maximum in the oxygenreduction activity for compositions of PdCo with a 50:50 ratio of atomicpercents. This optimum composition for the PdCo binary system wasindependent of the applied potential. In both cases the activity of theoptimum composition was much greater than for 100 At. % Pd and wasapproximately 4 times as great at 0.7 V and 7 times as great at 0.8 V.

BINARY EXAMPLE 2

The specific activities of the PdCo binary system toward oxygenreduction at 0.70 V vs. RHE were assessed using surface areas calculatedfrom the charge for oxidative stripping of CO adsorbed at Pd atoms inthe surface of the sample.

The charge for CO oxidative stripping was measured by saturating thesurface of the samples with CO by bubbling CO gas through theelectrolyte for 20 min while applying a potential of 0.1 V vs. RHE tothe array. Subsequently argon gas was then bubbled through theelectrolyte for 5 min. while maintaining the same applied potential torid the solution of any dissolved but unadsorbed CO. The array was thencycled between 0.0 V and 1.2 V vs. RHE for 4 cycles at 50 mV s.sup.−1.Using the cyclic voLtammogram, the anodic charge between 0.5 and 1.2 Vwas calculated for the first and fourth cycle. The charges associatedwith oxidation of a monolayer of adsorbed carbon monoxide (CO(ads)) weredetermined by subtracting the charge associated with the formation ofthe surface oxide layer (Q.sub.O) (fourth cycle) from the chargeassociated with the concomitant formation of the surface oxide layer andoxidation of the adsorbed carbon monoxide (Q_(CO+O)) (first cycle).Q _(CO) =Q _(CO+O) −Q _(O)  [1]

The carbon monoxide charges were converted to surface area estimates bydividing them by the constant 420.mu.Ccm.sup.−2. This is the value usedfor the determination of the real surface area of polycrystallineplatinum electrodes that also assumes complete coverage, 1 CO(ads) persurface Pt atom, and 2 e− for the oxidation of each CO(ads) to CO₂.

FIG. 3( a) shows the resultant surface areas estimates. The data showsthat there was a large increase in surface area of the samples forcompositions with less than 80 At. % Pd, this area increased reached amaximum at 50 At. % Pd. This area increase is thought to be due todissolution of Co from the binary alloy.

FIG. 3( b) shows the data of FIG. 2( a) divided by the surface areaestimates giving the specific current activities of the PdCo binarysystem for oxygen reduction. The specific activity data for the binaryhas a maximum activity at 70-80 At. % Pd. That there is still a maximumin activity following surface area correction is evidence that adding Coto Pd has an electronic effect that increases the activity of the alloytowards ORR over that observed for either Pd or Co in the pure state.

BINARY EXAMPLE 3

The difference between the peak current for the surface oxide reductionof the samples from the cyclic voltammograms recorded before and afterthe oxygen reduction screening experiments in 0.50 M HClO₄ (aq) is usedas a preliminary indication of sample stability. It is apparent fromFIG. 4 that the change in peak current is quite significant for theoptimum PdCo compositions. This suggests that the increase in activityof these PdCo materials comes at the expense of stability.

TERNARY EXAMPLE 1

The steady state ORR current for one of the PdCoAu arrays wasinvestigated. The steady-state oxygen reduction currents at (a) 0.70 V,(b) 0.75 V, and (c) 0.80 V vs. RHE are respectively shown in FIGS. 5(a), 5(b) and 5(c) in 0.5 M HClO₄ (aq). It is clear from this Figure thatthe region of highest activity is in the composition range of 40 to 60At. % Pd. The arrow in FIG. 5( a) shows the direction of the decreasingactivity that is brought about by the addition of increasing amounts ofAu to the alloys.

TERNARY EXAMPLE 2

The data of TERNARY EXAMPLE 1 was reworked as specific activities. Thespecific activities shown in FIG. 6 were calculated using the surfaceareas measured from the CO oxidative stripping charges (as explained inBINARY EXAMPLE 2). After converting the currents shown in FIG. 5 to thespecific current densities shown in FIG. 6, there remained a peak in ORRactivity that was attributable to a increase in the intrinsic activityof the material and was not a result of increased surface roughnessbrought about by partial (or complete) dissolution of the Co componentfrom the PdCoAu alloy surface.

TERNARY EXAMPLE 3

The steady state ORR currents for all 3 of the PdCoAu ternary arrayswere measured as well as the PdCo and PdAu binary arrays and Pd onlyarrays. The steady-state oxygen reduction currents at (a) 0.70 V, (b)0.75 V, and (c) 0.80 V vs. RHE are shown in FIG. 7 in 0.5 M HClO4(aq).It is clear from FIG. 7 that the region of highest activity is along thebinary axis and in the composition range of 40 to 60 At. % Pd and thatthere was a decrease in activity upon addition of increasing amounts ofAu to the alloys. However that the high activity of the alloy ismaintained at up to 20 At. % Au is an interesting result given that PdAubinary system is very inactive for compositions with <90 At. % Pd.

TERNARY EXAMPLE 4

The data of TERNARY EXAMPLE 3 was reworked as specific activities, againcalculated using the surface areas estimates from CO oxidative strippingcharges (as explained in BINARY EXAMPLE 2). The currents of FIG. 7 wereconverted to specific current densities, which are shown in FIG. 8.After conversion there remained a peak in ORR activity along the PdCobinary axis and in the composition range of 40 to 60 At. % Pd. Althoughthere was a decrease in activity upon addition of increasing amounts ofAu to the alloys, that the extremely high activity of the binary systemwas extended into the ternary phase space for up to 20 At. % Au, albeitat decreased activities, is an interesting result.

TERNARY EXAMPLE 5

The difference between the peak current for the surface oxide reductionof the samples from the cyclic voltammograms recorded before and afterthe oxygen reduction screening experiments in 0.50 M HClO₄ (aq) weretaken as a preliminary indication of sample stability, and plotted inFIG. 9. In FIG. 9( a) the data from the binary and ternary arrays areplotted together. It is clear that the change in surface oxide reductionpeak current was substantially larger for the binary alloy system (alsoshown in FIG. 4 for the binary system) than for the ternary systems andwas greatest for the binary at the most active composition. In FIG. 9(b) the same data for the ternary alloys only is plotted. By comparingthe current scales for the two plots, it is apparent that the change insurface oxide peak currents was greatly reduced for all ternary alloys.Also by comparing to FIG. 8, it is noted that the region of highactivity for ternary system extends beyond the region in FIG. 9 wherethere was greatest difference between the before and after peak current.Given that the difference between the peak currents for the surfaceoxide reduction measured before and after the steady state ORRexperiments is believed to indicate the stability of the samples, thisobservation is considered to highlight a region of the PdCoAu phasespace where there is both good specific activity and good stability(including corrosion resistance).

BINARY EXAMPLE 4

The steady state current for oxygen reduction was measured for an arrayof compositions for the PdFe binary system at 0.70 V vs. RHE. FIG. 10shows the steady state currents for oxygen reduction at 0.70 V vs. RHE.The data show there was a clear maximum in the oxygen reduction activityfor compositions of PdFe with between 50 and 60 At. % Pd.

TERNARY EXAMPLE 6

By analogy with the findings of TERNARY EXAMPLE 5 relative to BINARYEXAMPLEs 1 to 3 it is expected that the active regions found in BINARYEXAMPLE 4 can be developed into areas of activity and stability byaddition of Au as a further alloying element. Thus, it is predicted thatthe PdFeAu ternary alloy system, when prepared and tested as for thePdCoAu alloys described above, will also exhibit a region where there isboth good specific activity and good stability (including corrosionresistance). From the binary PdFe data, compositions of from 50-70 At. %Pd, 50-30 At. % Fe and 0-20 At. % Au will be of interest.

BINARY EXAMPLE 5 PdCr Binary: Oxygen Reduction Reaction

The steady state current for oxygen reduction was measured for PdCrbinary arrays at 0.70 V and 0.80 V vs. RHE in 0.5 M HClO₄ (aq). FIGS. 12and 13 show the steady state currents for oxygen reduction at 0.70 V and0.80 V vs. RHE respectively. The data show a clear maximum in the oxygenreduction activity for compositions of PdCr with between 55 and 65 At. %Pd.

BINARY EXAMPLE 6 PdCr Binary: Oxygen Reduction Reaction in Presence ofMethanol

The same electrochemical measurements were done in the presence ofmethanol. The steady state current for oxygen reduction was measured forPdCr binary system at 0.70 V and 0.80 V vs. RHE in 0.5 M HClO₄ (aq)+0.5M methanol. FIGS. 14 and 15 show the steady state currents for oxygenreduction at 0.70 V and 0.80 V vs. RHE respectively. The data show aclear maximum in the oxygen reduction activity for compositions of PdCrbetween 50 and 70 At. % Pd and also show that the binary system ismethanol tolerant.

TERNARY EXAMPLE 7 PdCoCr Ternary: Oxygen Reduction Reaction

The steady state current for the oxygen reduction reaction of PdCoCrarrays was investigated in 0.5 M HClO₄ (aq). For the entire binary andternary phase space investigated, the steady-state oxygen reductioncurrents at 0.70 V, 0.75 V and 0.80 V vs. RHE are respectively shown inFIGS. 16, 17 and 18.

It is clear from those Figures that the region of highest activity is inthe composition range of 40 to 80 At. % Pd. The high activity is alsomaintained at up to 30 At. % Cr.

TERNARY EXAMPLE 8 PdCoCr Ternary: Electrochemical Area

The electrochemical surface areas of the entire binaries (PdCo and PdCr)and ternary (PdCoCr) system were estimated from carbon monoxideoxidative stripping charges as explained in BINARY EXAMPLE 2. FIG. 19plots the data of surface area for the PdCo, PdCr binaries and PdCoCrternaries. The data show that there is a high surface area region at30-60 At. % Pd and 30-70 At. % Co and less than 20 At. % Cr. However amore stable region appears with the amount of Co decreased to 10-30 At.% and the amount of Cr increased to 20-40 At. %.

TERNARY EXAMPLE 9 PdCoCr Ternary: Specific Activity

The data of TERNARY EXAMPLE 7 was reworked as specific activities asexplained in TERNARY EXAMPLE 4. Plots of the steady-state oxygenreduction current densities for the PdCoCr films at 0.70 V and 0.80 Vvs. RHE in 0.5 M HClO₄ (aq) are shown in FIGS. 20 and 21 respectively.Upon addition of up to 40 At. % Cr the activity of PdCoCr alloys wasmaintained.

TERNARY EXAMPLE 10 PdCoW Ternary: Oxygen Reduction Reaction

The steady state current for the oxygen reduction reaction of PdCoWarrays was investigated in 0.5 M HClO₄ (aq). For the entire binary andternary phase space investigated, the steady-state oxygen reductioncurrents at 0.70 V and 0.80 V vs. RHE are shown in FIGS. 24 and 25respectively. It is clear from those figures that the region of highestactivity is in the composition range of 30 to 70 At. % Pd. The highactivity is also maintained at up to 20 At. % W.

TERNARY EXAMPLE 11 PdCoW Ternary: Electrochemical Area

The electrochemical surface areas of the entire binaries (PdCo and PdW)and ternary (PdCoW) system were estimated from carbon monoxide oxidativestripping charges as explained in BINARY EXAMPLE 2. FIG. 26 plots thedata of surface area for the PdCo, PdW binaries and PdCoW ternaries. Thedata show that there is a high surface area region for alloys containingmore than 20 At. % W. However a more stable region appears for alloyswith 30-80 At. % Pd, less than 40 At. % Co and less than 30 At. % W.

TERNARY EXAMPLE 12 PdCoW Ternary: Specific Activity

The data of TERNARY EXAMPLE 12 was reworked as specific activities asexplained in TERNARY EXAMPLE 4. Plot of the steady-state oxygenreduction current densities for the PdCoW films at 0.70 V and 0.80 V vs.RHE in 0.5 M HClO₄ (aq) are shown in FIGS. 27 and 28 respectively. Asillustrated in those figures, the most active alloys contain 20-60 At. %Pd, 30-70 At. % Co and 0-30 At. % W.

TERNARY EXAMPLE 13 Stability Measurement

Composition changes before and after the electrochemical screening ofarrays were monitored by EDS. FIG. 29 shows ternary plots for theeffective gain of Pd and Loss of Co and W of a PdCoW ternary array. Itclearly shows the instability of Co in the region of 30-60 At. % Pd,resulting in a corresponding effective increase of Pd atomic ratiocontent. Insignificant loss of W was observed for alloys containing lessthan 20 At. % W. These results agree with the electrochemical surfacearea measurements. Both measurements show that the most stable alloysare composed of more than 60 At. % Pd, less than 40 At. % Co and lessthan 20 At. % W.

TERNARY EXAMPLE 14 PdFeCr Ternary: Oxygen Reduction Reaction

The steady state current for the oxygen reduction reaction of PdFeCrarrays was investigated in 0.5 M HClO₄ (aq). For the entire binary andternary phase space investigated, the steady-state oxygen reductioncurrents at 0.70 V and 0.80 V vs. RHE are shown in FIGS. 32 and 33respectively. The data show that the most active catalysts have acomposition of 40-90 At. % Pd, 10-60 At. % Fe and 0-20 At. % Cr.

TERNARY EXAMPLE 15 PdFeCr Ternary: Electrochemical Area

The electrochemical surface areas of the entire binaries (PdFe and PdCr)and ternary (PdFeCr) system were estimated from carbon monoxideoxidative stripping charges as explained in BINARY EXAMPLE 2. FIG. 34plots the data of surface area for the PdFe, PdCr binaries and PdFeCrternaries.

TERNARY EXAMPLE 16 PdFeCr Ternary: Specific Activity

The data of TERNARY EXAMPLE 15 was reworked as specific activities asexplained in TERNARY EXAMPLE 2. Plot of the steady-state oxygenreduction current densities for the PdFeCr films at 0.70 V and 0.80 Vvs. RHE in 0.5 M HClO₄ (aq) are shown in FIGS. 35 and 36 respectively.The data show that the most active alloys are composed of 30-80 At. %Pd, 20-70 At. % Fe and 0-40 At. % Cr.

Reference Example 1 Rotating Disc Electrode Study of Thin Film Alloys

From data previously shown in the Binary and TERNARY EXAMPLEs, some thinfilm alloys were deposited on rotating disc electrodes and screened foroxygen reduction reaction. The compositions of the thin film alloys arelisted in Table 1 and were chosen to be in the most active and stableregions noted from the results represented in the Binary and TERNARYEXAMPLEs above.

Titanium discs (5 mm diameter) were used as substrates and were firstpolished then etched for 15 min in a HF/HNO.sub.3 solution (0.5 g NaF,4.5 mL of 70% HNO₃ and 10 mL H₂O). Thin films were then deposited ontothe titanium discs using the same physical vapour deposition methoddescribed earlier. The deposition of thin films onto the disks wascarried out without using wedges. Instead, appropriate deposition rateswere selected to ensure the deposition of continuous films of constant(and known) compositions across the area of the disc substrate. Thesubstrates were also rotated during deposition to ensure uniform filmproperties. Using this modified deposition technique it is possible toprepare up to 16 identical thin films on rotation disc electrodesubstrates. The compositions of all these thin films were confirmedafter deposition using EDS.

FIG. 38 shows the specific activity J.sub.k (real) (mass transportcorrected, electrochemical surface area specific) for each catalyst at0.85 and 0.90 V vs. RHE in saturated O.sub.2 solution of 0.5 M HClO₄(aq) at 25° C. Data were obtained from cyclic voltammetry measurementsin O₂ saturated solution at 20 mV s⁻¹ with a rotation rate of 900 rpm.Surface areas were obtained from carbon monoxide stripping experimentsas explained in BINARY EXAMPLE 2. The compositions of the Pd alloys werechosen to be those which show enhanced activity for oxygen reductionreaction in comparison to pure Pd. PdCoW alloys (51:41:08 At. %) exhibitthe highest activity, closely followed by the PdCo binary alloy. ThePdCoW alloys were more stable than the PdCo alloys. The costs ofspecific activity per market price (in $) at 0.85 V vs. RHE of all thestudied catalysts are summarised in FIG. 39. The price of the catalystsin $/g (at September 2006 market price) were calculated from theircompositions converted in weight % and the appropriate raw metal costs.The costs of materials processing into practical dispersed catalysts arenot included in this calculation. Similar cost values based on theresults obtained with pure platinum and palladium are also included forcomparison.

TABLE US-00001 TABLE 1 Composition of the alloys synthesised forrotating disc electrode studies. Alloy Composition (measured by EDS)PdCo 53 At. % Pd, 47 At. % Co PdCr 62 At. % Pd, 38 At. % Cr PdW 90 At. %Pd, 10 At. % W PdFe 67 At. % Pd, 33 At. % Fe PdCoCr 43 At. % Pd, 38 At.% Co, 19 At. % Cr PdCoW 51 At. % Pd, 41 At. % Co, 08 At. % Cr PdFeCr 40At. % Pd, 43 At. % Fe, 17 At. % Cr

The invention claimed is:
 1. A fuel cell comprising a metal alloycatalyst consisting essentially of the metals Pd, M1, and M2, where M1and M2 are different metals selected from Fe, Cr, and W.
 2. The fuelcell of claim 1, wherein M1 is Fe.
 3. A fuel cell comprising a catalystconsisting essentially of 40 to 60 atomic percentage (at %) Pd, 40 to 60at % Co, and up to 30 at % Cr.
 4. A fuel cell comprising a catalystconsisting essentially of 30 to 60 atomic percentage (at %) Pd, 30 to 70at % Co, and less than 20 at % Cr.
 5. A fuel cell comprising a catalystconsisting essentially of Pd, Co, and W.
 6. The fuel cell of claim 5,wherein the catalyst consists essentially of 60 atomic percentage (at %)or more Pd, 40 at % or less Co, and up to 20 at % W.
 7. The fuel cell ofclaim 1, wherein the catalyst consists essentially of Pd, Fe, and Cr. 8.The fuel cell of claim 7, wherein the catalyst consists essentially of40 to 80 atomic percentage (at %) Pd, 20 to 60 at % Fe, and up to 20 at% Cr.
 9. The fuel cell of claim 7, wherein the catalyst consistsessentially of 30 to 80 atomic percentage (at %) Pd, 20 to 70 at % Fe,and up to 40 at % Cr.
 10. A cathode for a fuel cell, the cathodecomprising a cathode support, and a metal alloy catalyst consistingessentially of the metals Pd, M1, and M2, where M1 and M2 are differentmetals selected from Fe, Cr, and W.
 11. A membrane-electrode assemblyfor a fuel cell, the assembly comprising a proton exchange membrane, ananode, and the cathode of claim
 10. 12. A fuel cell comprising at leastone membrane electrode assembly of claim
 11. 13. The fuel cell of claim1, wherein the fuel cell is a direct methanol fuel cell (DMFC) and thecatalyst is provided for the oxygen reduction reaction (ORR).
 14. A fuelcell comprising a ternary metal alloy catalyst, the alloy consistingessentially of 30 to 80 atomic percentage (at %) Pd, 10 to 30 at % Co,and 20 to 40 at % Cr.
 15. A cathode for a fuel cell comprising a cathodesupport and a metal alloy catalyst consisting essentially of Pd, Co, andW.
 16. The cathode of claim 15, wherein the metal alloy catalystconsists essentially of 60 atomic percentage (at %) or more Pd, 40 at %or less Co, and up to 20 at % W.
 17. A membrane-electrode assembly for afuel cell, the assembly comprising a proton exchange membrane, an anode,and the cathode of claim
 15. 18. A membrane-electrode assembly for afuel cell, the assembly comprising a proton exchange membrane, an anode,and the cathode of claim
 16. 19. A fuel cell comprising at least onemembrane electrode assembly of claim
 17. 20. A fuel cell comprising atleast one membrane electrode assembly of claim
 18. 21. A cathode for afuel cell comprising a cathode support and a metal alloy catalyst,wherein the metal alloy catalyst is a ternary alloy consistingessentially of 40 to 60 atomic percentage (at %) Pd, 40 to 60 at % Co,and up to 30 at % Cr; a ternary alloy consisting essentially of 30 to 60at % Pd, 30 to 70 at % Co, and less than 20 at % Cr; or a ternary alloyconsisting essentially of 30 to 80 at % Pd, 10 to 30 at % Co, and 20 to40 at % Cr.
 22. A membrane-electrode assembly for a fuel cell, theassembly comprising a proton exchange membrane, an anode, and thecathode of claim
 21. 23. A fuel cell comprising at least one membraneelectrode assembly of claim
 22. 24. The fuel cell of claim 3, whereinthe fuel cell is a direct methanol fuel cell (DMFC) and the catalyst isprovided for the oxygen reduction reaction (ORR).
 25. The fuel cell ofclaim 4, wherein the fuel cell is a direct methanol fuel cell (DMFC) andthe catalyst is provided for the oxygen reduction reaction (ORR). 26.The fuel cell of claim 5, wherein the fuel cell is a direct methanolfuel cell (DMFC) and the catalyst is provided for the oxygen reductionreaction (ORR).
 27. The fuel cell of claim 6, wherein the fuel cell is adirect methanol fuel cell (DMFC) and the catalyst is provided for theoxygen reduction reaction (ORR).
 28. The fuel cell of claim 12, whereinthe fuel cell is a direct methanol fuel cell (DMFC) and the catalyst isprovided for the oxygen reduction reaction (ORR).
 29. The fuel cell ofclaim 14, wherein the fuel cell is a direct methanol fuel cell (DMFC)and the catalyst is provided for the oxygen reduction reaction (ORR).